This invention relates to control of ocular development in general and, more particularly, to the treatment of the eye to prevent and/or arrest the development of myopia (nearsightedness). Approximately one of every four persons suffer from myopia, i.e., an elongation of the eye along the visual axis. In particular, myopia afflicts 10% to 75% of the youth of the world, depending upon race, geographic distribution and level of education. Myopia is not a trivial maldevelopment of the eye. In its pathologic form, the sclera continues to grow and as result the retina stretches and degenerates resulting in permanent blindness.
Inheritance, environmental forces such as diet, sun intake, and substantial eye use, etc., are but a few theories that have been postulated to explain the on-set of myopia. In that regard, preventive measures such as eye rest, eye exercise, eye glasses, contact lens and drug and surgical therapies have been proposed. However, these measures are neither ideal nor risk-free. The surgical therapies (e.g. corneal surgery using excimer lasers or conventional knives) attempted for this condition are drastic and often unsuccessful. Moreover, neither of the therapies (excimer lasers or conventional knives) are easily reversed or sufficiently predictable in their results. Complications from contact lens wear range from allergic reactions to permanent loss of vision due to corneal ulceration. Even with the complications associated with contact lens wear, there are roughly 24 million wearers in the United States, with the number expected to double in the next 3 years. While eyeglasses eliminate most of the medical risks listed above, they are not an acceptable option as evidenced by the contact lens wearers who tolerate the frustration of contact lens wear.
One particular drug therapy utilized in the treatment of myopia involves the use of cycloplegics. Cycloplegics are topically administered drugs that relax the ciliary muscle of the eye, which is the muscle that focuses the eye by controlling lens dimensions. The classic cycloplegic drug is the belladonna alkaloid atropine, available for over a century. Atropine is a long-acting non-specific antimuscarinic agent that antagonizes the action of the neurotransmitter acetylcholine (ACh) at autonomic effector cells innervated by postganglionic cholinergic nerves of the parasympathetic nervous system. However, use of atropine, is impractical in that it causes mydriases (increase of pupil size) and its action on the ciliary muscle to inhibit ocular focusing impairs near visual work like reading. There is strong evidence that the receptors in the iris and ciliary muscle responsible for the side effects of atropine are of the M3 subtype. Additionally, studies have shown that muscarinic receptors in the retina of a variety of non-human species are comprised of m1, m2 and m4 subtypes. Accordingly, a muscarinic antagonist with low m3 activity would be efficacious in prevention of the development of myopia without the undesirable side effects associated with the use of atropine.
There is now substantial evidence to link the posterior part of the eye, specifically image quality at the retina and hence an extension of the nervous system, to the postnatal regulation of ocular growth. There is significant evidence of myopia in an eye that is subjected to retinal image impairment. It has been shown that axial myopia can be experimentally induced, in either birds or primates, in an eye in which the .retina is deprived of formed images, e.g., by suturing the eyelids or wearing an image diffusing goggle. The experimental myopia induced in birds or primates such as monkeys mimics, in many respects, the axial myopia of humans.
Thus, the phenomenon of an animal's vision process apparently contributes to the feedback mechanism by which postnatal ocular growth is normally regulated and refractive error is determined. This indicates that this mechanism is neural and likely originates in the retina. R. A. Stone, et al. have found a method of controlling the abnormal postnatal growth of the eye of a maturing animal, which comprises controlling the presence of a neurochemical, its agonist or antagonist, which neurochemical is found to be changed under conditions during maturation leading to abnormal axial length. See U.S. Pat. Nos. 4,066,772 and 5,284,843. Therein it is disclosed that retinal concentrations of dopamine were found to be reduced during such image deprivation and the ocular administration of a dopamine-related agent, e.g., apomorphine, a dopamine agonist, was found to inhibit or actually prevent the axial enlargement of the eye under conditions ordinarily leading to such enlargement.
There have also been recent advances made in the understanding of the cholinergic nervous system and the receptors thereto. Cholinergic receptors are proteins embedded in the wall of a cell that respond to the chemical acetylcholine. Particularly, it is now known that the cholinergic receptors are subdivided into nicotinic and muscarinic receptors and that the muscarinic receptors are not all of the same type. Recent literature indicates that there are at least five types of cholinergic muscarinic receptors (types m1 through m5). Receptors of type m 1 are those present in abundance and thought to be enriched in the brain neural tissue and neural ganglia. The other receptors are concentrated in other tissues such as the heart, smooth muscle tissue or glands. While many pharmacological agents interacting with muscarinic receptors influence several types, some agents are known to have a major effect on a single type of receptor with relative selectivity and other agents can have a relatively selective effect on a different single receptor. Still other agents may have a significant effect on more than one or even all types of receptors.
It is known, for example, that pirenzepine, (Gastrozepin, LS 519) 5,11-Dihydro-11-[4-methyl-1-piperazinyl)acetyl]-6H-pyrido[2,3-b]benzodiaze pin-6-one, and its dihydrochloride are anticholinergic, antimuscarinic, and relatively selective for M1 receptors. See U.S. Pat. No. 5,122,522. It is also known that 4-DAMP (4-diphenylacetoxy-N-methylpiperadine methiodide) is a relatively selective antagonist for smooth muscle (ordinarily called M3 type but variously called type M2 or M3, as the current classification of receptors is in flux). Pirenzepine, being primarily an M1 antagonist, inhibits axial elongation, but is far less effective at pupil dilation than atropine or another cycloplegic agent. This makes it possible to suppress the development of myopia without dilating the pupil and paralyzing the accommodation activity of the ciliary muscle. Additionally, the administration of a drug topically into the eye of a developing child for a long period of time makes it desirable to have a minimal likelihood of sensitization of the eye. Pirenzepine and atropine test positive in sensitization assays and this is an undesirable side effect.